Method for production of SOI transistor device and SOI transistor device

ABSTRACT

A wafer bonding method for forming a SOI structure comprising the steps of bringing wafers into proximity in a state with one wafer a slight, substantially uniform clearance away from the other wafer and pressing one point of at least one wafer of the two wafers against the other wafer. In another aspect of the invention, there is provided a method of positioning for photolithography using an alignment mark portions and/or a vernier portions formed on a SOI substrate, which comprises the step of removing semiconductor layer portions corresponding to the alignment mark portions and/or the vernier portions. In further another aspect of the invention, there is provided a new DRAM semiconductor device formed by using SOI structure, which comprises a new pattern of a strage node formed longitudinally along a word line. Further, there is provided a new DRAM semiconductor device formed by using SOI structure, which comprises a unique strage node having a conductive side wall.

This application is a continuation of application Ser. No. 08/213,535filed Mar. 16, 1994, now abandoned, which is a divisional of applicationSer. No. 08/063,641 filed May 20, 1993, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for production of asilicon-on-insulator (SOI) transistor device.

More specifically, it relates to a wafer bonding method. The presentinvention may be used for bonding two substrates to form a cladsubstrate. For example, it may be utilized for bonding two semiconductorwafers to form a composite semiconductor wafer.

An SOI structure is useful for semiconductor devices and otherelectronic components. In this technique, a silicon portion is placed onan insulator portion and various semiconductor elements are fabricatedin the silicon portion. As one means for formation of an SOI structure,there is known the technique of bonding a separate substrate to asilicon substrate having insulator layer on the surface with theinsulator and then polishing the silicon substrate to form siliconportions on the insulator layer. This is generally called a bonding SOI(for the bonding technique, see for example Nikkei McGraw-Hill "NikkeiMicrodevices", March 1988, p. 84.)

Further, the present invention relates to a SOI transistor device, inparticular a semiconductor device having capacitor storage nodes buriedin a thin film semiconductor layer using the SOI structure.

2. Description of the Related Art

The SOI structure is used for various applications. For example, byforming elements in the semiconductor portion on the insulator material,a semiconductor device with good element isolation can be obtained fromthe beginning.

An example of the SOI structure to which the present invention isapplied will be explained below.

Numerous proposals have been made regarding SOI structure semiconductordevices. There are also numerous means for formation of the same. Whenapplying the present invention, any of these may be used. One method offormation is to form a so-called bonding SOI structure. The formation ofan SOI structure will be explained referring to FIGS. 1A to 1E taking asan example this technique (see Extended Abstracts on the 21st Conferenceon Solid State Devices and Materials, Tokyo, 1989, pp. 89-92, M.Hashimoto et. al., "Low Leakage SOI MOSFETs Fabricated Using a waferBonding Method".)

As shown in FIG. 1A, one surface of a silicon substrate 1 (in general,use is made of a silicon wafer with a high degree of flatness, referredto here as the "substrate A") is patterned by photolithography oretching to form depressions of a depth of 1500 angstroms or less. Next,an SiO₂ film is formed on this surface by chemical vapor deposition(CVD) etc. so as to form an insulator 2, thereby obtaining a structureof a silicon substrate 1 with an insulator 2 on one side. The insulator2 is formed as an uneven film as illustrated due to the topography ofthe patterned silicon substrate 1. Further, a polycrystalline siliconfilm etc. is formed as a bonding layer 3 on the insulator 2 by CVD etc.to a thickness of about 5 μm. The result is the structure of FIG. 1A.The polycrystalline silicon film forming the bonding layer 3 is used forformation of a highly smooth bonding surface when bonding anothersubstrate (the substrate 4 shown by "B" in FIG. 1C) in a later step.

Next, the surface of the bonding layer 3 is polished to flatten it andobtain a highly smooth surface (FIG. 1B). By this, the bonding layer 3(polycrystalline silicon film) is reduced to a thickness of 3 μm orless.

Another substrate 4 (referred to here as the "substrate B") is placed inclose contact with the polished surface of the bonding layer 3. The twosurfaces are bonded by pressing them closely together. As a result thebonded structure shown in FIG. 1C is obtained. In general, it is saidthat a firm bond is achieved by the hydrogen bonding caused by theaction of water or hydroxyl groups interposed between the two surfaces.This assembly is usually then heated for heat bonding to form a strongbonding. The bonding strength is generally at least 200 kg/cm² andsometimes as high as 2000 kg/cm². As the other substrate 4 (substrate B)which is bonded use is usually made of the same type of siliconsubstrate as the substrate 1 (substrate A). This is because a heatingstep is often applied after the bonding so there is a danger of troubleif the heat expansion and other physical properties are not equal. Ifthis were not a problem, then in the; related art shown in FIG. 1, forexample, the other substrate 4 would function only as a support, sowould not have to absolutely be a silicon substrate. However, if formingelements in the other substrate 4, (substrate B) which is bonded thesubstrate would have to be a semiconductor substrate allowing theformation of elements.

Next, the substrate 1 is ground to reduce the silicon portion of thesubstrate 1 to a thickness of about 5 μm or less to form a residual filmand obtain the structure shown in FIG. 1D. FIG. 1C to FIG. 1E show theassembly turned upside down from FIG. 1A and FIG. 1B. This is becausethe assembly is turned upside down to put the substrate 1 at the top forgrinding to obtain the structure shown in FIG. 1D and for the followingselective polishing step.

Next, selective polishing is performed. Precision polishing is performedhere until the insulator 2 is exposed. As a result, as shown in FIG. 1E,a structure is obtained with silicon portions 10 present on andsurrounded by the insulator 2. The silicon portions 10 form the SOIfilm. In this structure with silicon portions 10 present in an insulator2 (SOI structure), elements are fabricated in the silicon portions 10.As shown in FIG. 1E, the silicon portions 10 are surrounded by theinsulator 2, so the elements are completely isolated from the verystart.

FIGS. 1A to 1E showed the silicon portions 10 enlarged for clarifyingthe illustration. In actuality, there are a large number of fine siliconportions 10.

When forming the bonding SOI structure as described above, sometimes airbubbles are entrained in the bonding interface. Explaining this usingFIG. 1C, when bonding the polycrystalline silicon film 3 surface on thesubstrate 1 with the substrate 4, air bubbles sometimes are entrained atthe interface. This is because when bonding flat surfaces of thesubstrates, there are numerous points of contact and air bubbles easilyare entrained. The air bubbles are of a size, for example, of about 0.5μm to 5 min.

The bonding surfaces are not in close contact at the portions where theair bubbles are entrained so sufficient bonding is not achieved andpeeling easily occurs. For example, after the grinding the surface ofFIG. 1D, the thickness t₁ of the film including the substrate 1 on thesubstrate 4 is 4 to 20 μm, usually about 5 to 10 μm or so, so if thereare air bubbles present, peeling easily occurs. This sometimes becomes asource of contamination and can have a serious impact on the fabricationof the elements. For example, the wafers are sometimes scratched by thecontamination during the polishing. The peeling due to the air bubblesforms contamination and impairs the reliability of the elements. In thestate of FIG. 1E, the thickness t₂ of the film on the silicon portion 10side of the substrate 4 is even thinner, about 3 to 4 μm. If peelingoccurs here, it becomes a source of contamination in the same way andsometimes ends up resulting in a defective element.

Therefore, it is necessary to ensure that no air bubbles form at thebonding portion of the two substrates.

In the past, to prevent the formation of air bubbles between the twosubstrates to be bonded when forming a bonding SOI, as shown in FIG. 2Ato 2C, the technique was employed of using a first support 13 forsupporting one substrate 12 and a second support jig 14 for supportinganother substrate 11, for example, a semiconductor wafer, using thesecond support jig 14 to make the surface of the supported substrate 11convex, and bonding with the convexity facing the substrate 12 (theconvexity is shown in an exaggerated manner to make the illustrationclearer).

In the above type of technique, however, since one substrate 12 is madeto be convex for bonding, there is some expansion and contraction of thepattern on the substrate 12, though slight. That is, when the bondingsurface of the substrate is convex in shape, in the case of a substratehaving a device pattern, the bonding is performed with elongation of thepattern due to the curvature of the convexity. Therefore, there is apossibility of misalignment of the pattern pitch and pattern deviationin the succeeding exposure step. Further, even if the second support jig14 supporting the substrate 11 is comprised of, for example, a vacuumchuck, it does not necessarily mean that the substrate 11 can besupported with a good convexity. Further, when bonding the substrates toform a flat composite substrate, that is, when bonding the substratesfrom the state of FIG. 2B to the state of FIG. 2C, a good, tight bond isnot achieved and therefore sometimes pattern elongation or contractionwill end up occurring. As a result, with the above described bondingtechnique, where one of the substrates was made convex, it was difficultto keep up with the demands of the recent technique--where patterns arebeing increasingly miniaturized.

Next, the other problem related to prior SOI technique will beexplained.

The element isolation pattern of an SOI structure obtained by thebonding method is formed, for example, by photolithography of the SOIwafer. There are problems in the photolithographic step, however. Thatis, in general, positioning marks known as alignment marks are formed atseveral locations on the chip so as to enable positioning during thephotolithography step. Further, separate so-called "vernier" portionsare generally formed at one position for each mark on the chip for thepurpose of confirmation of the positioning. FIG. 3 and FIG. 4 illustratethe alignment marks in the case of positive photolithography. FIG. 4shows the sectional shape of an alignment mark portion, while FIG. 3shows the structure from a plane view. In FIG. 3, the case of positivephotolithography is shown. The portion 21 in FIG. 3 is a SOI layerforming the semiconductor portion, while the portion 22 is the SiO₂forming the insulating material (note that in the case of negativephotolithography, 21 designates conversely SiO₂ and 22 designates theSOI layer). In FIG. 3, "O" indicates the center (origin) of thecoordinate system.

FIG. 4 shows the sectional structure of the alignment mark in the caseof making a gate window. In FIG. 4, 27 is the semiconductor portion (Si)in this example, while 24 is the insulator (SiO₂) surrounding thissemiconductor portion. Reference numeral 25 is a polycrystalline siliconlayer, while 26 is a top layer of tungsten silicide etc. Here, thesemiconductor portion 27 forming the SOI layer has a level difference ofas much as 40 nm from the SiO₂ portion of the insulating material (seeFIG. 4). When using this mark to form a resist pattern on a film with anextremely high reflectance, the exposure apparatus must be able toaccurately detect a level difference of as little as 40 nm. Thisdetection is extremely difficult in practice.

FIG. 5 shows the waveform of the alignment signal in the related art. Asillustrated there, the signal-to-noise ratio (S/N) ratio is extremelysmall and the chance of picking up a false signal is high. In FIG. 5,"O" shows the signal peaks.

Use is made of a vernier portion even when measuring the alignment afterexposure. Visual detection of the vernier is difficult for the samereasons, however, so accurate measurement of the alignment is notpossible.

Next, an explanation will be made of an example of a dynamic randomaccess memory (DRAM) using an SOI structure according to the relatedart.

Along with the reduction in size of DRAM memory cells and their higherdensity, use has been made of the SOI structure to secure the requiredstorage capacity. Development has been made of DRAMs and othersemiconductor devices with capacitors buried under a semiconductorlayer. The key portions of a DRAM or other semiconductor device usingsuch an SOI structure is shown in FIGS. 6A and 6B.

As shown in FIGS. 6A and 6B (wherein FIG. 6A is a sectional view alongthe line A--A in FIG. 6B), in this semiconductor device, a semiconductorlayer 32a constituted by silicon is formed on top of an insulating filmlayer 42 to produce an SOI structure. On the semiconductor layer 32a arelaid word lines 36a and 36b over a gate insulating layer. On top of thisare laid an interlayer insulating layer and bit lines 40. The bit lines40 are connected to the semiconductor layer 32a or the semiconductorlayer 32b through bit line contacts 39 made in the interlayer insulatinglayer. The word lines 36a and 36b serve also as gate electrodes. Achannel portion 38 is formed at the semiconductor layer 32a or 32bpositioned below the same. As a result, a DRAM memory cell transistor isformed at this portion.

Under the insulating film layer 42 is buried a capacitor storage node44. The storage node 44 and the semiconductor layer 32a are connectedthrough a contact formed in the insulating film layer 42. Under thestorage node 44 is laid a cell plate layer 48 through a capacitorinsulating film layer 46. In this semiconductor device, the storage node44, the insulating film layer 46, and the cell plate 48 constitute thecapacitor.

In a semiconductor device of such a structure, the area occupied by thecapacitor is made large to increase the capacity of the capacitor. Also,the level difference of the capacitor does not have a detrimental effecton the wiring patterning.

However, in a semiconductor device of an SOI structure now underdevelopment, as shown in FIG. 6A and 6B, the layout pattern of thestorage node 44 is positioned at the layers below the channel portion 38of the semiconductor layer 32a or 32b, so there were the followingproblems:

The potential of the storage node 44 changes from the ground level tothe power source voltage depending on the state of the data held. If thestorage node 44 is positioned directly under the channel portion 38, thechanges in potential of the storage node 44 will affect the channelportion 38 and cause a change in its potential state and fluctuations inthe threshold voltage. Therefore, it becomes difficult to maintain theoptimal threshold voltage and there is a danger of a reduction of thewriting voltage or a deterioration of the holding of the data.

Note that the layout pattern of the storage node

shown is one for the layout of storage type capacitors employed for a AMDRAM, 16M DRAM, etc.

Further, proposal has been made of a DRAM cell structure with a storagenode buried under a thin film semiconductor layer using an SOIstructure.

FIG. 7 is a schematic sectional view of a DRAM cell using an SOIstructure according to the related art.

In this conventional DRAM cell using an SOI structure, as shown in FIG.7, an insulating film layer 68 is laid under the thin film semiconductorlayer 58. The insulating film layer 68 had contact holes 69 formed in itnear the thin film semiconductor layer 58. Under the insulating filmlayer 68 were formed storage nodes 60. The capacitor storage nodes 60were connected to the thin film semiconductor layer through the contactholes 69. Under the storage nodes 60 was laid a cell plate layer 66through a capacitor insulating film 64. The capacitor storage nodes 60,insulating film layer 64, and cell plate layer 66 constitutedcapacitors. Note that in the figure, 54a and 54b are word lines and 56is a bit line.

In such a structure of a DRAM cell, it is possible to generate moresurface area of the side walls and obtain a desired storage capacity byincreasing the height (depth) of the storage nodes 60. Further, sincethe pattern for formation of the storage nodes 60 is a pattern whichremoves overlapping margin of the contact holes 69, the area of thecells can be expected to be considerably reduced.

When making such a DRAM cell of the related art, however, as shown inFIG. 8, an insulating layer 68 is laid on the semiconductor substrate 67for formation of the semiconductor layer 58 shown in FIG. 7, contactholes 69 are formed, a polycrystalline silicon film forming the storagenodes 60 is laid, then the polycrystalline film is etched by reactiveion etching (RIE) etc. to form the node formation holes 61 and thestorage nodes 60 are formed. In the related art, however, due to theoveretching at the time of etching for formation of the storage nodes,the polycrystalline silicon in the contact holes 69 and the surface ofthe thin film semiconductor substrate 67 were even etched and there wasthe danger of formation of the grooves A.

To achieve a higher density, if the pattern for formation of the storagenodes 60 is made a pattern for removing the overlapping margin of thecontact holes 69, during the RIE overetching for formation of thestorage nodes 60, part of the polycrystalline silicon film filled in thecontact holes 69 is also exposed to the etching. As a result, thepolycrystalline silicon is etched even in the contact holes 69. If theamount of overetching can be controlled so that the etching stops at theinside of the contact holes 69, there would be no problem, but if itreaches the surface of the substrate 67 and forms grooves "A" there, itcreates defective shapes, leak deficiencies, etc. at the time ofprocessing the substrate in later steps. In particular, the higher theheight of the storage node 60 is made to secure the capacitor capacity,the greater the etching error becomes and the more difficult control ofthe overetching becomes and the easier defects are to occur. Forexample, recently, the thickness of the polycrystalline silicon film hasto be at least about 2 μm. If the etching precision in RIE etc. is 10percent or so with respect to the thickness of the polycrystallinesilicon film, when etching the polycrystalline silicon film to form anode formation hole 61, the etching error is at least ±200 nm. On theother hand, the thickness of the insulating film layer 68 is about 200to 400 nm. In this way, the thickness of the insulating film layer 68 isnot sufficiently great with respect to the etching error and there is adanger of formation of grooves A in the surface of the substrate 67.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a method ofproduction of an SOI transistor device etc. wherein the method ofbonding of the substrates can prevent the occurrence of air bubbles,gives an excellent bond, and is free from expansion or contraction ofthe pattern on the substrates and other problems.

A second object of the present invention is to provide a method forproduction of an SOI transistor etc. wherein the method of positioningin the SOI structure and the method of confirmation of the positioningenable accurate detection of the alignment signal using alignment marksand enable accurate evaluation of deviation.

A third object of the present invention is to provide a semiconductordevice of an SOI structure having storage nodes which reduces the effectfrom the storage nodes to the channel portions, has cell transistorswith stable threshold voltages, and, when used as a DRAM, has excellentdata holding characteristics and data writing characteristics.

A fourth object of the present invention is to provide a semiconductordevice, and a process for production of the same, which is free fromformation of grooves on the semiconductor substrate due to overetchingwhen etching to form the storage nodes in the process of producing thesemiconductor device, has a small cell area, and has capacitors with alarge storage capacity.

In order to achieve the first object of the present invention, there isprovided a substrate bonding method for forming a SOI structurecomprising the steps of bringing substrates into proximity in a statewith one substrate a slight, substantially uniform clearance away fromthe other substrate and pressing one point of at least one substrate ofthe two substrates against the other substrate.

In accordance with the substrate bonding method of the presentinvention, it is possible to prevent the generation of air bubbles,achieve a bond in a good state, and bond the substrates free fromproblems such as pattern elongation or contraction.

In order to achieve the second object of the present invention, there isprovided a method of positioning for photolithography using an alignmentmark positions and/or vernier positions formed on a SOI substrate, whichcomprises the steps of removing semiconductor layer portionscorresponding to the alignment mark portions and/or the vernier portionsand making and confirming alignment for photolithography on the SOIsubstrate using the alignment mark portions and/or vernier portions.

In accordance with the method of positioning for photolithography of thepresent invention, it is possible to provide a positioning method and apositioning confirmation method in an SOI structure which enablesaccurate detection of the alignment signal using the alignment marks andenables accurate evaluation of misalignment.

In order to achieve the third object of the present invention, there isprovided a new semiconductor device (ex. DRAM) formed by using SOIstructure, which comprises a new pattern of a strage node formedlongitudinally along a word line.

In accordance with the new semiconductor device having the new patternof the strage node of the present invention, the storage nodes are nolonger positioned at the bottom layer side of the channel portion of thesemiconductor layer. As a result, the effect of the storage nodes on thechannel portion is lowered and the threshold voltage of the transistorin the cell is stabilized. When this semiconductor device is used as aDRAM, the data holding characteristics and the data writingcharacteristics are improved.

In order to achieve the fourth object of the present invention, there isprovided a new semiconductor device (ex. DRAM) formed by using SOIstructure, which comprises a unique strage node having a conductive sidewall.

In accordance with the new semiconductor device comprising a uniquestrage node having the conductive side wall, it is possible toeffectively prevent the formation of grooves by overetching to a surfaceof a semiconductor layer of the SOI structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A to FIG. 1E show an example of the process of formation of an SOIstructure according to a first related art,

FIG. 2A to FIG. 2C show a bonding method for preparing a substrate forforming an SOI structure device according to the first related art,

FIG. 3 shows alignment marks according to a second related art showingthe background art,

FIG. 4 is a sectional view of the alignment mark showing the secondrelated art,

FIG. 5 is a view showing the waveform of an alignment signal accordingto the related art shown in FIG. 3 and FIG. 4,

FIGS. 6A and 6B are respectively a sectional view and a plane view ofkey portions of a DRAM device of an SOI structure according to a thirdrelated art,

FIG. 7 is a schematic sectional view showing a DRAM according to afourth related art,

FIG. 8 is a schematic sectional view showing a process for production ofthe DRAM shown in FIG. 7,

FIGS. 9A to 9D show a wafer bonding method for forming an SOI deviceaccording to a first embodiment of the present invention,

FIG. 10 shows a wafer bonding method for forming an SOI device accordingto a second embodiment of the present invention,

FIG. 11 shows a substrate after wafer bonding obtained by the method ofFIG. 10,

FIG. 12 shows the flow of a process for the formation of a device usingan SOI substrate according to a third embodiment of the presentinvention,

FIG. 13 is a view of the waveform of the mark signal (X direction,before resist coating) of the third embodiment,

FIG. 14 is a view of the waveform of the mark signal (Y direction,before resist coating) of the third embodiment,

FIG. 15 is a view of the waveform of the mark signal (X direction, afterresist coating) of the third embodiment,

FIG. 16 is a view of the waveform of the mark signal (Y direction, afterresist coating) of the third embodiment,

FIG. 17 and FIG. 18 are respectively a sectional view and a plane viewof key portions of a DRAM according to a fourth embodiment of thepresent invention,

FIG. 19 is a schematic view of key portions showing the process ofproduction of a DRAM of the fourth embodiment of the present invention,

FIG. 20 is a plane view of a DRAM according to a fifth embodiment of thepresent invention,

FIG. 21 is a plane view of a DRAM according to a sixth embodiment of thepresent invention,

FIG. 22 is a schematic sectional view of a DRAM using an SOI structureaccording to a seventh embodiment of the present invention,

FIGS. 23A to 23F are views showing the flow of the process for formingthe SOI structure DRAM shown in FIG. 22,

FIG. 24 is a sectional view of an SOI structure DRAM according to aneighth embodiment of the present invention, and

FIGS. 25A to 25C are views showing the flow of the process for formingthe SOI structure DRAM shown in FIG. 24.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, an explanation will be made of specific embodiments of thepresent invention with reference to the drawings. Note that the presentinvention is not limited to these embodiments and includes modificationswithin the scope of the claims.

First, an explanation will be made of a first embodiment of the presentinvention using FIGS. 9A to 9D. This embodiment relates to a bondingmethod of semiconductor wafers for formation of a clad semiconductorwafer having a pattern formation SOI structure. According to thisembodiment, in the bonding of the substrates, it is possible totremendously reduce the expansion and contraction of the substrates andthe entrainment of air bubbles.

First, the substrates 71 and 74 to be bonded are subjected in advance toa hydrophilic treatment on their surfaces so that full use may be madeof the hydrogen bonding force of the substrates. They are used in astate free from dust and other deposits. The hydrophilic treatment maybe performed by (1) performing fluorine treatment to give clean Sisurfaces to the substrates 71 and 74, then immediately (2) performingtreatment by a mixed solution of ammonia and hydrogen peroxide water((2) alone also possible). It is believed that the bonding force becomeshigher by this treatment since the substrate surfaces become OH rich.

As shown in FIG. 9A, use is made of a flat vacuum chuck 80 to set thesubstrate 74. A vacuum is created as shown by the arrow 83. Thesubstrate is fixed level in position by this and then the next operationis performed. Note that the vacuum chuck 80 used in this example iscomprised of the air-permeable member 81 which bears the substrate 74 onits surface to support the same and a support 82 which surrounds andsupports this member.

Next, as shown in FIG. 9B, the substrate 71 is brought into close,uniform proximity to the substrate 74 with a clearance of several μm.This is done by having the substrate 71 held by a vacuum chuck fromabove in the same way as the substrate 74, then releasing the top vacuumstate at a position where the two substrates 71 and 74 are separated byat least a clearance where they do not contact each other (for example,1 mm). As a result, the substrate 71 falls toward the substrate 74, butwhen the distance becomes several μm, the substrate 71 may be made tofloat on the substrate 74 with a slight clearance 77 due to the viscousresistance of air. This is believed to contribute to the electrostaticand other action between the substrates 71 and 74.

Bonding step is performed from this state as shown in FIG. 9C. That is,a push rod with a contact area of 1 to 10 mmφ (in this example, a goodresult can be achieved using a push rod of 5 mmφ, for example) is usedas a pressing means 75 to apply a load of 0.5 to 5N (in this example, agood result can be achieved with 2N, for example) to any point of thesubstrate 71, such as near the center or at the periphery etc.,whereupon bonding is started by the hydrogen bonding force, as shown byreference numeral 76 in FIG. 9C.

After this, the substrate 71 is allowed to stand on the flat substrate74 in a natural state until the entire surface is adhered on its own(about 3 to 10 seconds in the case of a wafer of a diameter of 5inches).

FIG. 9D shows the state after the entire surface has adhered. The vacuumflow 83 is released in this state and the bonding is thus completed.

According to this embodiment, since the adhesion occurs along the flatsubstrate 74, there is no expansion or contraction of the substrate.Further, since self adhesion is used, the air is naturally expelled, soit is possible to reduce the entrainment of air bubbles.

In this embodiment, the substrates 71 and 74 are kept slightly apart bybringing the substrate 74 close to the substrate 71 in the direction ofgravity and making use of the elasticity of air between the substrates71 and 74, but the invention is not limited to this. For example, it isalso possible to keep them slightly apart by forcibly applyingelectrostatic force.

Next, an explanation will be made of a second embodiment of the presentinvention.

This embodiment relates to a bonding method of substrates to prepare asemiconductor element.

For example, when using a flat surface vacuum chuck for attracting onesubstrate as in the first embodiment, the studies of the presentinventors show that when the other substrate 71 (substrate in freestate) is bonded (hydrogen bonds), the bonding proceeds with a slightelongation at the portion of the reference numeral 76 shown in FIG. 9C.

As a result, when the bonding ends and the substrate is detached fromthe vacuum chuck, a springback force of the elongation occurs andwarping occurs causing the free state side substrate to become concave.This surface clearly was bonded while elongated. Therefore, with a finepattern, there is a danger that the pattern pitch will no longer bealigned.

In this embodiment, the bonding step is performed while compensating foramount of elongation in advance by making the fixed surface convex inshape so as to ensure the same elongation of the fixed side substrateand the free side substrate. Due to this, the elongation is eliminatedin the end and bonding free of elongation is achieved.

As mentioned above, there is the following problem when setting asubstrate on a vacuum chuck having a flat attachment surface, creating avacuum, then bringing another substrate close in a free state, pressingone point, by a push rod, and thus starting the hydrogen bonding. Thatis, since the substrates are bonded by the hydrogen bonds against theviscous resistance of air, the bonding ends up occurring while theinterface is slightly elongated with a convex curvature. This can beunderstood from the fact that the substrate warps to a concave surfaceafter the bonding.

As opposed to this, in the present embodiment, the following isperformed as shown in FIG. 10. The amount of elongation at the time thesubstrate 91 is elongated and bonded is, for example, 10 to 20 ppm orso, so as shown in FIG. 10, the fixed surface of the substrate 94 isgiven a convex spherical shape 99 etc. of a curvature of about the sameextent as the elongation.

The substrate 91 is bonded with the substrate fixed by a fixing jig 100which has such a convex curved surface and fixes and supports thesubstrate 94. Therefore, after the bonding as shown in FIG. 11, thesubstrates 91 and 94 are bonded by substantially the same amount ofelongation, so the elongation is cancelled. Therefore, the bonding stepis achieved free of deviation.

In this embodiment, a convex spherical surface was employed as thisenables the elongation to be reduced the most, but another convex curvedsurface may be employed as well.

According to the present invention, it is possible to prevent thegeneration of air bubbles, achieve a bond in a good state, and bond thesubstrates free from problems such as pattern elongation or contraction.

Next, an explanation will be made of a third embodiment of the presentinvention.

This embodiment applies to the production of an SOI structuresemiconductor device.

In this embodiment, the positioning and the confirmation of positioningduring the formation of the SOI structure are performed in accordancewith the process sequence shown in the flow chart of FIG. 12. That is,in the embodiment, first, the SOI substrate formation step 111 isperformed. Here, the previous explained bonding technology is used toobtain a substrate with an SOI structure. Next, in a photolithographystep 112, alignment is performed with respect to an element isolationpattern and the resist is opened at the alignment mark portions andvernier portions. Next, an Si etching step 113 is performed to removethe semiconductor portions (Si portions) of the alignment mark portionsand the vernier portions. A resist peeling step 114 is then performedand then positioning is performed using the alignment mark portions, andthe positioning is confirmed using the vernier portions.

More specifically, the following processes are usually used for an SOIwafer with an element isolation pattern:

(1) Step of preparation of element isolation pattern (formation ofinsulation portions)

(2) Step of preparation of clad SOI wafer (bonding and polishing)

(3) Step of preparation of device

The termination of the above (2) and the input to (3) is referred to asline reinput. In the present embodiment, during this line reinput, theSOI layers of the alignment mark portions and the vernier portions ofthe SOI wafer with the element isolation pattern are etched and asufficient level difference is ensured in the pattern. The SOI layer atthe other portions (for example, the element formation portions) ismasked by the resist and not etched. Therefore, a photolithography stepaligning the lower element isolation pattern is required, but at thattime it is sufficiently possible to detect the waveform of the alignmentsignal for the following reasons:

(1) The surface of the pattern is constituted by two substances, SiO,and Si, having different reflectances.

(2) The range of tolerance with respect to deviation in alignment islarge (for example, at this time, fine alignment of 0.15 μm is notrequired, about 2 μm is sufficient).

As a result, as shown from FIG. 13 to FIG. 16, a clear mark signalwaveform is obtained. FIG. 13 shows the waveform of the detected marksignal in the X direction before resist coating, FIG. 14 shows thewaveform of the detected mark signal in the Y direction before resistcoating, FIG. 15 shows the waveform of the detected mark signal in the Xdirection after resist coating, and FIG. 16 shows the waveform of thedetected mark signal in the Y direction after resist coating.

By this, in this embodiment of the present invention, a sufficient leveldifference of the mark portions can be secured, so a signal with asufficient S/N ratio can be obtained.

According to the present invention, as mentioned above, it is possibleto provide a positioning method and a positioning confirmation method inan SOI structure which enables accurate detection of the alignmentsignal using the alignment marks and enables accurate evaluation ofmisalignment.

Next, a fourth embodiment of the present invention will be explainedwith reference to the drawings.

FIG. 17 and FIG. 18 are a sectional view and plane view of key portionsof a DRAM according to the fourth embodiment of the present invention.

The fourth embodiment shown in FIG. 17 and FIG. 18 is an example of theapplication of the present invention to a DRAM. In this embodiment, thesemiconductor layers 122a and 122b are formed by a predeterminedpattern. On top of this are laid word lines 126a and 126b through a gateinsulating layer 124. The semiconductor layers 122a and 122b are formedby the bonding SOI method and are comprised of monocrystalline siliconetc. Note that in the illustration, only two lines of semiconductorlayers 122a and 122b are shown, but a large number are actually formedin accordance with the number of the memory cells.

The gate insulation layer 124 laid on the semiconductor layers 122a and122b is for example formed by heat oxidation of a semiconductor layer orCVD and is comprised of a silicon oxide layer etc. The word lines 126aand 126b formed on the gate insulating layer 124 are for examplecomprised of a polycrystalline silicon film formed by the CVD method,for example. Patterning is performed in a direction substantiallyperpendicularly intersecting the semiconductor layers 122a and 122bpositioned under the same. A channel portion 128 is formed in thesemiconductor layer portion of the position cutting across thesemiconductor layers 122a and 122b. That is, the word lines 126a and126b serve also as gate electrodes. The gate electrodes constituted bythe word lines 126a and 126b, the gate insulation layer 124, the channelportion 128, and the semiconductor layer 122a or 122b where thesource-drain portion is formed, constitute cell transistors.

On top of the gate electrode layers 126a and 126b are laid an interlayerinsulating layer comprised of a silicon oxide layer formed by the CVDmethod, for example. On top of this are laid the bit lines 130. The bitlines 130 are formed by a polycrystalline silicon film formed by the CVDmethod, for example. The bit lines 130 are connected to thesemiconductor layers 122a and 122b of the memory cells through the bitcontacts 129 formed in the interlayer insulating layer.

Under the semiconductor layers 122a and 122b there is an insulating filmlayer 132. The insulating film layer 132 is comprised, for example, of asilicon oxide layer. Under the insulating film layer 132 is buried acapacitor storage node 140. The storage node 140 and the semiconductorlayer 122a or 122b are connected through a contact 133 formed in theinsulating film layer 132. Under the storage node 140 is laid a cellplate layer 138 through a capacitor insulating thin film layer 136. Thestorage node 140 is comprised of polycrystalline silicon, for example.The capacitor insulating thin film layer 136 is comprised, for example,of a laminate of a silicon oxide thin film and a silicon nitride thinfilm etc. The cell plate layer 138 is comprised, for example, ofpolycrystalline silicon.

In this semiconductor device, the storage node 140, the insulating thinfilm layer 136, and the cell plate 138 constitute a capacitor.

In the present embodiment, the layout pattern of the storage node 140 ismade a longitudinal pattern naming along the word lines 126a and 126b.Further, the sectional width of the storage node 140 is madesubstantially equal to the width of the word lines 126a and 126b.Further, to make the area of the plane side of the storage node as largeas possible and to increase the capacity of the capacitor, as shown inFIG. 18, the node 140 is formed narrower and longer to reach theposition close to the layout pattern of the adjoining semiconductorlayers 122a and 122b. As a result, the capacity of the capacitor of thisembodiment is not reduced as compared with the capacity of the capacitorof the storage node shown in FIGS. 6A and 6B.

By using the layout pattern of the storage node 140 of the presentembodiment, the area directly under the channel portion 128 of the celltransistor is free of any storage node 140 and associated fluctuatingpotential, but is located opposite a cell plate layer 138 which isstable in potential, so there is no longer any fluctuation of thethreshold voltage of the cell transistor due to the state of holding ofdata. Therefore, in the DRAM of the present embodiment, the data holdingcharacteristics and the data writing characteristics are improved.

Further, if the layout pattern of storage node 140 according to thepresent embodiment is used, as shown in FIG. 18, the area of contactbetween the contact 133 and the storage node 140 becomes larger comparedwith the example shown in FIG. 6B and the contact between thesemiconductor layers 122a and 122b and the storage node 140 becomesreliable.

Next, an explanation will be made of a process for production of theDRAM shown in FIGS. 17 and 18.

FIG. 19 is a schematic view of key portions showing an example of theprocess of production of the DRAM. As shown in FIG. 19, first, anelement isolation level difference 180a is formed at the surface of asilicon semiconductor substrate 180. On the surface, a silicon oxidefilm comprising the insulating film layer 132 is formed by CVD etc. Thethickness of the silicon oxide layer is not particularly limited, butfor example is about 300 nm.

After this, a contact 133 is formed by a predetermined pattern in theinsulating film layer 132 and a polycrystalline silicon film forming thestorage node 140 is deposited by the CVD method. The thickness of thepolycrystalline silicon film forming the storage node 140 is notparticularly limited, but for example is about 700 nm. After this, thepolycrystalline silicon film is etched in a predetermined pattern toobtain the storage node 140.

After this, a capacitor insulating thin film layer 136 comprised of alaminate of silicon oxide film and a silicon nitride film etc. is formedon the surface of the storage node 140 and the insulating film layer132. A polycrystalline silicon film for formation of the cell platelayer 138 is formed by the CVD method etc. on the surface of thecapacitor insulating thin film layer 136. The thickness of thepolycrystalline silicon film is not particularly limited, but forexample is about 400 nm. On top of this, a polycrystalline silicon filmis further deposited to about 5 μm, the surface is polished to about 3μm to flatten it, then the support substrate 172 is bonded by waferbonding method or other means. The support substrate 172 is constituted,for example, by a silicon wafer.

Next, the semiconductor substrate 180 is ground from the rear surface.Finally, polishing is performed to the position L using the siliconoxide film constituting the insulating film layer 132 as a stoppersurface and therefore a thin semiconductor layer 122a of about 100 nm isobtained. After this, the assembly is turned upside down so that thesupport substrate 172 side is at the bottom, then as shown in FIG. 17, agate insulating layer 124, the word lines 156a and 156b, the interlayerinsulating layer, and the bit line 160 are successively formed on thesurface of the semiconductor layer 122a.

FIG. 20 is a plane view of a DRAM according to a fifth embodiment. Asshown in FIG. 20, the sectional width of the storage nodes 210a and 210aare greater than the width of the word lines 196a and 196b and thecorner portion of one storage node 210a approaches the corner portion ofthe other storage node 210b.

In this embodiment, the storage node 210a can be arranged away from thechannel portion 198 of the adjoining semiconductor layers 192a and 192bby a certain margin, so even if misalignment occurs, the positioning ofthe storage node directly under the channel 198 can be effectivelyprevented. Despite this, the area occupied by the storage node 210a isnot reduced compared with the embodiment shown in FIG. 18 and asufficient capacitor capacity can be realized.

Further, in this embodiment, the corner portions of the storage nodes210a and 210a are in close proximity, but since there is an effect ofthe refracted light at the time of exposure, it is possible to easilyseparate the two by patterning and therefore the two never shortcircuit.

The longitudinal sectional shape of the storage nodes in the directionof the word lines 196a and 196b is not necessarily limited to arectangular shape.

FIG. 21 is a plane view of a DRAM according to a sixth embodiment. Asshown here, the shape may be a triangular shape having a bottom sidenaming along the direction of the word lines 226a and 226b. In thisembodiment too, a similar action as in the above embodiment is obtained.

The semiconductor device of the present invention need not be used foronly a DRAM, but can be used for another: semiconductor device having anSOI structure with storage nodes.

As explained above, according to the present invention, in asemiconductor device of an SOI structure having storage nodes, thestorage nodes are formed at the top layer side of the semiconductorlayer in a longitudinal fashion along the word lines formed so as tosubstantially perpendicularly intersect the semiconductor layer, so thestorage nodes are no longer positioned at the bottom layer side of thechannel portion of the semiconductor layer. As a result, the effect ofthe storage nodes on the channel portion is lowered and the thresholdvoltage of the transistor in the cell is stabilized. When thissemiconductor device is used as a DRAM, the data holding characteristicsand the data writing characteristics are improved.

Next, an explanation will be made of an eighth embodiment of the presentinvention.

FIG. 22 is a schematic sectional view of a DRAM according to the seventhembodiment of the present invention.

In this embodiment, the thin film semiconductor layer 258 is formed by apredetermined pattern word lines 254a and 254b are laid on top of itover a gate insulating layer 257. The thin film semiconductor layer 258is formed by the bonding SOI method and is comprised of monocrystallinesilicon etc. Note that in the figure, only the thin film semiconductorlayer 258 formed with the channel portion of the word line 254a isillustrated, but in actuality a large number are formed in apredetermined pattern in the two dimensional directions in accordancewith the number of the memory cells.

The gate insulating layer 257 formed on the thin film semiconductorlayer 258 is comprised for example of a silicon oxide layer etc. Theword lines 254a and 254b formed on the gate insulating layer 257 arecomprised by a polycrystalline silicon film formed by the CVD method.The word lines 254a and 254b serve also as the gate electrodes of atransistor and have a source region and drain region formed at the twosides. The word lines 254a and 254b are patterned in the directionsubstantially perpendicularly intersecting the thin film semiconductorlayer 258 positioned at the lower layer. A channel portion is formed atthe portion of the semiconductor substrate positioned cutting across thethin film semiconductor layer 258.

On the word lines 254a and 254b are laid an interlayer insulating layer255 comprised by a silicon oxide layer formed by the CVD method. On topof this are formed a bit line 256. The bit line 256 passes through thebit contact hole formed in the interlayer insulating layer 255 toconnect with the thin film semiconductor layer 258 of the memory cells.The thickness of the interlayer insulating layer 255 is not particularlylimited, but for example is 300 nm.

Underneath the thin film semiconductor layer 258 is laid an insulatingfilm layer 268. The insulating film layer 268 has formed in it a contacthole 286 near the thin film semiconductor layer 258. Under theinsulating film layer 268 is formed a storage node 279 by apredetermined pattern. In this embodiment, the storage node 279 iscomprised of a conductive layer 262b comprised by a polycrystallinesilicon film etc. and a side wall 262a comprised by a polycrystallinesilicon film formed at the side portion of a dummy layer 218. The dummylayer 218 is comprised for example of a silicon oxide layer, thethickness of which is not particularly limited, but for example is about1.2 μm. At the outer periphery of the side wall 262a constituting thestorage node 279 is laid a cell plate layer 266 through a capacitorinsulating thin film layer 264. The capacitor insulating thin film layer264 is comprised for example of a silicon oxide thin film and a siliconnitride thin film etc. The cell plate layer 266 is for example comprisedof polycrystalline silicon, the thickness of which is for example about50 nm. Under the cell plate layer 266 is laid a flattening layer 278 anda support substrate 263. The flattening layer 278 is comprised of apolycrystalline silicon film, while the support substrate 263 iscomprised for example of a silicon wafer etc.

In this semiconductor device, the capacitor storage node comprised bythe side wall 262a and the conductive layer 262b, the insulating thinfilm layer 264, and the cell plate 266 constitute a capacitor.

In this embodiment, by making the thickness of the dummy layer 278greater, it is possible to make the height of the side wall 262a greaterand possible to increase the capacitor area. Therefore, the capacitorcan be given a large storage capacity with a small cell area. Further,due to the reasons discussed later, during etching for forming thestorage node 279, the semiconductor substrate forming the semiconductorlayer 258 is never overetched.

Next, an explanation will be made of a process for producing the DRAM ofthe seventh embodiment of the present invention.

shown in FIG. 23A, first, an element isolation level difference 201 isformed on the surface of a silicon semiconductor substrate 238. Asilicon oxide film forming the insulating film layer 268 is formed bythe CVD method etc. The thickness of the silicon oxide film is notparticularly limited, but is about 300 nm.

After this, a contact hole 286 to expose the surface of thesemiconductor substrate 238 is formed in the insulating film layer 268.

Next, as shown in FIG. 23B, a conductive layer 262b comprised of apolycrystalline silicon film for example is deposited by the CVD methodetc. The thickness of the conductive layer 262b is not particularlylimited, but is for example 400 nm. After this, a dummy layer 218comprised of a different material from the above conductive layer 262bis deposited. The dummy layer 218 is for example formed by an SiO₂ layeretc. formed by the CVD method and is not particularly limited inthickness, but is for example 1.2 μm.

Next, as shown in FIG. 23C, the dummy layer 218 is etched by RIE using amixed gas of CHF₂ and O₂ for example and a first node formation hole 232is formed. The dummy layer 218 is comprised of a material different fromthe conductive layer 262b, so when etching the dummy layer 218 using theabove mixed gas, it becomes possible to secure a high etchingselectivity with the conductive layer 262b formed under the dummy layer218. Therefore, it becomes possible to form the first node formationhole 232 in the dummy layer 218 without etching the conductive layer262b much at all.

Next, as shown in FIG. 23D, the conductive layer 262b under the firstnode formation hole 232 is etched by RIE using chlorine type plasma etc.so as to form a second node formation hole 234. At this time thethickness of the conductive layer 262b is less than the thickness of thedummy layer 218, so the amount of overetching for forming the secondnode formation hole 234 need only be small and therefore the conductivelayer 262b inside the contact hole 234 is not all removed and thesurface of the semiconductor substrate 238 is not etched to formgrooves.

Next, as shown in FIG. 23E, a conductive layer of a polycrystallinesilicon film etc. of a thickness of 60 nm, for example, is deposited bythe CVD method on the surface of the dummy layer 218 and then is etchedso as to form the side wall 262a at the inner periphery of the first andsecond node formation holes 232 and 234. The side wall 262a and thepolycrystalline silicon film 262b constitute the capacitor storage node.

Next, as shown in FIG. 23F, a silicon nitride film of about 7 nm isformed at the outer periphery of the side wall 262a forming the storagenode 279, the surface of this is oxidized to about 2 nm, and thus acapacitor insulating thin film layer 264 is formed.

After this, a polycrystalline silicon film forming the cell plate layer266 is formed on the surface of this capacitor insulating thin filmlayer 264 to a thickness of about 50 nm using the CVD method etc.Further, a flattening layer 278 comprised of a polycrystalline siliconfilm etc. is deposited on top of this to a thickness of about 5 μm. Thesurface is polished about 3 μm to flatten it, then a support substrate263 is bonded. The support substrate 263 is comprised of a siliconwafer, for example.

Next, the assembly is turned upside down so that the support substrate263 is at the bottom and the semiconductor substrate 238 is ground fromthe rear surface. Finally, this is ground to the position "L" using thesilicon oxide layer constituting the insulating thin film 268 as thestopper surface to thus obtain the thin film semiconductor layer 258 offor example 100 nm. After this, as shown in FIG. 22, the gate insulatingfilm 257, word lines 254a and 254b, the interlayer insulating layer 255,and the bit line 256 are successively formed on the surface of the thinfilm semiconductor layer 238 to obtain a DRAM cell.

FIG. 24 is a schematic sectional view according to an eighth embodimentof the present invention.

In the DRAM according to the eighth embodiment of the present invention,unlike with the previous seventh embodiment, the dummy layer 218 isremoved after the side wall 262a is formed. Also, a capacitor insulatingfilm layer 264 and cell plate layer 266 are formed at the outerperiphery and inner periphery of the side wall 262a, so the capacitorarea of a storage node 269 of this embodiment becomes about twice thatof the seventh embodiment. Note that the same numerals in the eighthembodiment as the numerals in the seventh embodiment designate the samecomponents.

Next, an explanation will be made of the process of production for theDRAM cell of the eighth embodiment.

After the same steps as the process for producing a semiconductor deviceaccording to the above seventh embodiment, as shown in FIG. 23E, theside wall 262a is formed, then as shown in FIG. 25A, patterning isperformed based on the inverted data of the storage node to form apredetermined shape of a resist mask 290 in the first node formationhole 232 and the second node formation hole 234.

Next, as shown in FIG. 25B, fluorine treatment etc. is used to etch thedummy layer 218 and remove it. Due to this, a side wall 262a of apolycrystalline silicon film is left at the inner periphery of the firstnode formation hole 232.

When forming the resist 290, as shown in FIG. 25B, due to the exposureerror, the resist 290 sometimes is formed covering the dummy layer 218,but since etching is performed using fluorine treatment etc., the dummylayer 218 is completely etched and removed.

Next, the resist 290 shown in FIG. 25B is etched to remove the same.

After this, as shown in FIG. 25C (turned upside down from FIG. 25B), thepolycrystalline silicon film forming the capacitor insulating thin filmlayer 204 and cell plate layer 266 are successively laid. On top ofthis, further, a flattening layer 278 comprised of a polycrystallinesilicon film etc. is deposited to a thickness of about 5 μm. The surfaceis polished to about 3 μm to flatten it, then the support substrate 263is bonded. The support substrate 263 is comprised for example of asilicon wafer.

Next, the rear surface of the semiconductor substrate 238 is ground toobtain a thin film semiconductor layer in the same way as the seventhembodiment. After this, a gate insulating film, word lines, aninterlayer insulating layer, and a bit line are successively formed onthe surface of the thin film semiconductor layer to obtain the DRAM cellshown in FIG. 24.

According the present invention, a conductive side wall is formed at theside wall of the dummy layer and this side wall is used as part of thestorage node. Further, when etching to form the storage node, first afirst node formation hole is formed in the dummy layer and a second nodeformation hole is formed in the conductive layer below it. That is, thenode formation holes are etched in two stages, whereby the thickness ofthe conductive layer may be made smaller and the error at the time ofetching of the conductive layer may be made relatively small. As aresult, it is possible to effectively prevent the formation of groovesby overetching to the surface of the semiconductor substrate.

Further, in particular, according to the present invention, in which thedummy layer remaining at the inside of the conductive side wall isremoved and a capacitor insulating layer and cell plate layer are formedat the outer and inner peripheries of the side wall, the capacitorcapacity can be further increased.

What is claimed is:
 1. A semiconductor device comprising:a storage nodeformed below and in direct contact with a semiconductor layer having apredetermined pattern, said storage node being formed so as to extendlongitudinally along beneath a word line which is formed above saidsemiconductor layer and which extends in a direction which issubstantially perpendicular to a direction in which said semiconductorlayer extends, said semiconductor layer being connected with a bit linewhich is formed above said word line, by way of a bit contact; aninsulating layer formed over a top of said storage node, said insulatinglayer covering both upper and lower sides of said semiconductor layer;and a stepped portion formed on an upper portion of said storage nodewhich extends through said insulating layer in a manner wherein saidstepped portion engages an edge portion of said semiconductor layer. 2.A semiconductor device as set forth in claim 1, wherein a sectionalwidth of said storage node is substantially equal to a width of saidword line.
 3. A semiconductor device as set forth in claim 1, wherein asectional width of said storage node is greater than a width of saidword line and a corner portion of said storage node is close to a cornerportion of an adjacent storage node.
 4. A semiconductor device as setforth in claim 1, further comprising:an insulating thin film layer whichis formed so as to cover a predetermined portion of said storage node;and a cell plate layer which is formed so as to enclose a portion ofsaid storage node which is covered by said insulating thin film layer.5. A semiconductor device as set forth in claim 1,wherein saidsemiconductor layer is bonded to said insulating layer using a SOIbonding method.
 6. A semiconductor device as set forth in claim 1,wherein said storage node is made of a first type of silicon materialand said semiconductive layer is formed of a second type of siliconmaterial.
 7. A semiconductor device as set forth in claim 6, whereinsaid first type of silicon material is polycrystalline silicon and saidsecond type of silicon material is monocrystalline silicon.
 8. Asemiconductor device as set forth in claim 1, further comprising:a dummylayer which is formed of a material which is different from said storagenode, said dummy layer being formed about an exterior surface of apredetermined lower portion of said storage node; means defining ahollow portion in a predetermined lower portion of said storage node; aninsulating thin film layer which covers a wall of said hollow portion;and a cell plate layer which is formed below said dummy layer and whichfills said hollow portion of said storage node, said cell plate layerbeing separated from said storage node by said insulating thin film. 9.A semiconductor device as set forth in claim 1, wherein a sectionalshape of said storage node is triangular and wherein the triangle has aside which extends in a direction of said word line.
 10. A semiconductordevice comprising:a semiconductor layer which has a predeterminedpattern and which is formed on an insulating layer, said semiconductorlayer being formed beneath a word line and a bit line and connected withthe bit line by a bit contact; a capacitor storage node which is formedbeneath and in direct contact with said semiconductor layer, saidcapacitor storage node having no part which is higher than an uppersurface of said semiconductive layer; a capacitor insulating thin filmlayer formed so as to cover a portion of said storage node; and a cellplate layer formed under said insulating thin film layer; said storagenode having an outer peripheral side wall surrounded by a dummy layercomprised of a material which is different from that of the storagenode.
 11. A semiconductor device comprising:a semiconductor layer of apredetermined pattern formed on an insulating layer, said semiconductivelayer being formed beneath and spaced from a word line and a bit lineand connected with the bit line by a bit contact; a capacitor storagenode formed under said semiconductor layer, said storage node includinga first wall portion having a first inner periphery and a first outerperiphery and a second wall portion having a second inner periphery anda second outer periphery; a dummy layer formed between said first andsecond wall portions in a manner to contact the first inner periphery ofsaid wall portion and the second inner periphery of said second wallportion; a capacitor insulating thin film layer formed under saidstorage node so as to cover the first outer periphery of said first wallportion and the second outer periphery of said second wall portion; acell plate layer formed under said insulating thin film layer.
 12. Asemiconductor device for use in a SOI DRAM comprising:a bit line; afirst word line; a second word line; a continuous semiconductor layerhaving a predetermined pattern; a channel portion formed in saidsemiconductor layer at a location immediately below and essentiallywithin a width of said first word line, said first and second word linesbeing formed above said semiconductor layer, said first and second wordlines being respectively separated from said channel portion and saidsemiconductor layer by a gate insulating layer, said channel portionforming part of a memory cell transistor and said first and second wordlines acting as gate electrodes; an insulating layer on which saidsemiconductor layer is formed; and a storage node formed below and indirect contact with said semiconductor layer, said storage node beingformed so as to extend through said insulating layer and also extendlongitudinally beneath and essentially within a width of said secondword line, said storage node being formed at a location which isseparated from said channel portion and in a manner which minimizes thecoupling capacitor effect of said storage node on said channel portion,said storage node being formed so that no part of said storage nodeextends above said semiconductor layer.
 13. A semiconductor device asset forth in claim 12, further comprising a stepped portion formed on anupper portion of said storage node which projects through saidinsulating layer and which engages an edge portion of said semiconductorlayer; said semiconductor layer being bonded to said insulating layer bya SOI bonding method.
 14. A semiconductor device as set forth in claim12, wherein no part of the storage node overlaps the channel portionwhich is formed in the semiconductor layer and which forms part of amemory cell transistor.
 15. A semiconductor device comprising:a cellplate layer; a first insulating layer formed on said cell plate layer; asemiconductor layer formed on said first insulating layer; a secondinsulating layer formed over said semiconductor layer; a channel portionformed in said semiconductor layer; a first word line formed in saidsecond insulating layer so as to extend over said channel portion, saidinsulating layer, said channel portion and said first word line defininga field effect transistor; a second word line formed in said secondinsulating layer so as to extend over said semiconductor layer; and acapacitor node formed in said cell plate layer, said capacitor nodeextending upwardly through said first insulating layer to establishcontact with said semiconductor layer, said capacitor node being formedessentially within a width of second word line, said capacitor node andsaid channel portion being arranged with respect to one another in amanner which is effective to establish a spatial relationship whereinany coupling capacitor effect therebetween is attenuated, said capacitornode being arranged with respect to said semiconductor layer so that nopart of said capacitor node is located above said semiconductor layer.16. A semiconductor device as set forth in claim 15, further comprisinga bit line, said bit line being formed on said second insulating layerand arranged to have a bit contact with extends through said secondinsulating layer and contacts said semiconductor layer at a location.17. A semiconductor device as set forth in claim 15, wherein said bitcontact contacts said semiconductor layer at a location wherein saidfirst word line is interposed between the location and said second wordline.